This invention relates to tunable polarizers in an optical fiber transmission systems wherein the polarizer device comprises an optical fiber integrated with the optical fiber of the transmission system. More particularly it is directed to such devices that have means for dynamic tuning to yield polarization loss equalizers.
Polarization effects in fiber-optic transmission systems are well known. The effects of unequal loss between modes that propagate with different states of polarization have been studied, and several approaches to compensating for this effect have been proposed. A fiber-optic transmission line is ideally polarization insensitive, so that lightwave signals in any state of polarization (SOP) propagate with similar attenuation, pulse shape and velocity. In reality, a transmission system contains several components and fibers that attenuate light in different SOPs to varying degrees. This is called polarization dependent loss (PDL), and is deleterious to the performance of a communication link because random fluctuations in signal SOP translate into random fluctuations in the power of the signal. Moreover, concatenation of several devices and fibers with varying amounts of PDL makes the overall PDL time-dependent, because of random mode-coupling phenomena throughout the transmission line. Mitigating this effect would entail adding components that offer selective attenuation to different SOPs of light. Thus, a device with predetermined PDL would serve to negate the PDL of a series of components, yielding a transmission line that has no PDL.
A variety of compensation means using static elements have been proposed. These operate on the assumption that the intrinsic polarization dependency of the system remains constant. Should new polarization effects arise, the system requires reconstruction of the compensating elements to restore equalization. Dynamic changes in the system polarization may arise from a variety of factors, including for example, physical handling and bending of the optical fiber for splicing etc., and changes in temperature of the system ambient. To address dynamic system effects requires a dynamic or tunable compensation device.
Moreover, since PDL of a series of devices is time-dependent, effective compensation requires a device with tunable PDL. Adjustable PDL compensators have been used to decrease the 2% power penalty tail in a 10 Gb/sec system from 6.5 dB to 1.5 dB (See L. S. Yan et. al, ECOC-2001, We p. 38). In addition, PDL in the presence of polarization mode dispersion (PMD) not only increases the system penalty but also mitigates the effectiveness of polarization-mode-dispersion (PMD) compensation schemes. Thus, a tunable PDL controller (called PDLcon, hereafter) that equalizes the PDL of a transmission system is also important in PMD compensation schemes.
The building block for a PDLcon is a polarizer that attenuates light in selected SOPs. Several existing fiber-device technologies may be used to achieve this. Existing fiber polarizers fall into two broad categories. The first is a set of devices that comprise a fiber-waveguide structure in which an attenuating or birefringent material interacts with the evanescent mode to selectively attenuate one SOP, while not perturbing the orthogonal SOP of light. These devices may be implemented using a single crystal, liquid crystal or birefringent polymer cladding around the core of the fiber, or by polishing one side of the fiber and metallizing it. An alternative to adding some attenuating or birefringent material is to design the fiber-waveguide such that only one of the two orthogonal SOPs is guided by the fiber. All these devices are broadband and offer strong polarization extinction, but their PDL cannot be tuned as would be required of a PDLcon. The other class of devices comprises long period fiber-gratings (LPG) written in birefringent fibers. LPGs have been written in birefringent fibers to yield polarizers with up to 30-dB polarizability. The PDL obtained is thus limited to the bandwidth of the resonance of the grating (1-2 nm). The bandwidth may be broadened by chirping the grating, but an inherent trade-off between bandwidth and tunability of such gratings makes them unsuitable as PDLcon devices.
Typical prior art variable or tunable PDL devices comprise free-space components. For example, a variable PDL device disclosed in U.S. Pat. No. 5,740,288 comprises polarization beam-splitters, combiners and liquid crystal cells. The orientation of the liquid crystal cells serve to selectively attenuate one of the SOPs by controllable amounts, thus yielding a variable PDL device. U.S. Pat. No. 6,347,164 discloses a variable PDL device that comprises polarization beam-splitters, combiners and planar lenses, where the PDL is varied by tilting the planar lenses with respect to light incident on it. Both of these prior art devices involve careful assembly of several free-space optical components. Free space optics (as contrasted with integrated optics wherein the signal remains in a common medium for processing) are costly, lossy and may not be reliable in challenging applications such as under-sea transmission systems.
Thus, there exists the need for an integrated optical device that can offer variable PDL over a large spectral bandwidth while maintaining the low loss, low cost and reliable characteristics of all-fiber devices. Ideally, the integrated optical device would comprise an optical fiber integrated system.
This invention resides in part on the recognition of the characteristics of known grating elements, in this case long-period fiber gratings (LPGs), and their utility for the applications just described. LPGs offer coupling between co-propagating modes of a fiber, and have been used to realize wavelength selective attenuation filters. LPGs are traditionally narrow-band devices, and while they offer strong (e.g.  greater than 20 or 30 dB) loss, the spectral width of the coupling is typically limited to a range spanning from, e.g., 0.5 nm to 2 nm. On the other hand, if a fiber waveguide is engineered to yield two modes with identical group velocities a broadband spectrum is obtained in which the strength (or loss), rather than resonant wavelength, varies when tuned. This approach yields strong broadband loss-filters, in which the LPG couples the core mode to a specific higher-order cladding mode whose group velocity equals that of the core mode. Such gratings are called turn-around-point (TAP) LPGs.
The current invention is based on the discovery that the properties of TAP LPGs can be realized in birefringent fibers, i.e. in fibers where the propagation constants of the core and/or cladding modes are different for different SOPs of light. This yields a device that exhibits broadband loss of varying amounts for different SOPs of input light. Thus, the building block for a broadband in-fiber polarizer is realized. Further, tuning mechanisms are applied to vary the relative coupling magnitudes for two orthogonal SOPs of light in the device. This yields a polarizer in which the polarization is dynamically tuned. Hence, a TAP LPG written in a birefringent fiber yields a tunable PDL device.
A variety of grating tuning mechanisms are applicable to this device. The period of the birefringent TAP LPGs can be dynamically altered by changing the strain applied on the LPG (the strain may be applied by piezoelectric packages, simple motion control housings or magnetically latchable materials). Alternately, the propagation constants of the modes can be perturbed by temperature, the electro-optic effect, the nonlinear optic effect, or any other means that modifies the refractive index profile of the birefringent TAP fiber.
The all-fiber PDLcon of the invention offers such advantages over discrete free-space device as low loss, low manufacturing cost, reliability, and ease of tuning. In addition, it does not require complex polishing and film deposition steps needed for making the static fiber polarizers of the prior art.